The zeolite 13X is a synthetic zeolite the natural form of the structure is Faujasite, so the structure of both is called FAU (IZA three letter nomenclature).
The charge compensation cation in 13X is Na+. The diameter of the cavity in zeolite FAU is 13 Angstrom.
Short: zeolite 13X = NaX it has the same structure then Faujasit
I am not sure what the 1.5 mm gives because I do not found the certain product at the merck Homepage. But I think it is the diameter of the zeolite beads, which are pressed from the powder.
Yes, 13X Molecular Sieve is same as NaX Zeolite. Reference: Nanostructured Pt/C electrocatalysts with high platinum dispersions through zeolite-templating @ http://www.sciencedirect.com/science/article/pii/S1387181106005646